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## Session 102

We’re back with discrete questions from full-length 10. We’re looking at enzymes, ATP, antioxidants, and buoyancy. Check it out and don’t forget to subscribe.

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### [02:13] Questions and Passages in Random

Bryan explains that there’s always four sets of discrete questions and the section always ends with a set of them. Other than that, it shifts around the bunch. It can either be two passages and then discrete or a set of three and then discrete. So they’re basically given in random.

### [03:03] Question 27

An object weighs 150 grams in air, 75 grams when fully submerged in water, and 60 grams when fully submerged in an unknown fluid. What can be concluded about the specific gravity of the unknown fluid and the order of layers when water is mixed with the unknown fluid, assuming the unknown fluid is immiscible in water.

- (A) The specific gravity of the unknown fluid is 1.2 and the water will be the top layer.
- (B) The specific gravity of the unknown fluid is 0.83 and the unknown fluid will be the top layer.
- (C) The specific gravity of the unknown fluid is 0.56 and the unknown fluid will be the top layer.
- (D) The specific gravity of the unknown fluid is 1.5 and the water will be the top layer.

*Bryan’s Insights:*

Take note here that the thing has weight in water, meaning it sinks to the bottom. So like when you put a little bathroom scale on the bottom of your pool and drop this thing in, it does sink to the bottom. Hence, we know this thing is more dense than water. And given that 2:1 ratio, it’s probably twice as dense as water, which is 1.

Buoyant force is that upward force that’s suspending it and it’s proportional to the density of the fluid. So if this fluid is able to push up on this object a little bit better, then it’s more dense than water.

Obviously, B and C are out because they’re both less than 1.

Looking at proportional change in the apparent weight, it goes from 75 to 60 so it loses 1/5 of its apparent weight, which is 0.2. So it’s 1.2, which is a little bit denser than water. Whereas 1.5 is already pretty dense, otherwise the apparent weight would be chopped all the way down to 50 grams and would lose half as much of its apparent weight.

### [06:55] Useful Tips When Tackling Physics

Bryan points out that just because there are equations in physics doesn’t mean we always have to start doing calculations. Sometimes, if you can just picture the scenario in your head, imagine pulling and pushing and sinking, then ask yourself that if this stuff is super floaty then it’s not very dense. So imagine visually what’s being described here.

You can often pick answer choices that are reasonable without even applying any particular algebra, without doing any particular equation.

Especially with physics, students find it to be so abstract so they think it has something to do with variables and letters, do some math, get their answer and that’s it. But Bryan explains this stuff is the real world. Like this is the physical reality that your brain has evolved to operate it. So use that monkey brain and those monkey fingers. Imagine pushing and pulling and speeding up and slowing down, turning and floating, etc. This is the real physics that shows up on the MCAT. So you’ve got to trust that intuition sometimes as you may not have time to do all these in-depth calculations.

### [08:40] Question 28

Which of the following is most likely to be the enzyme labeled x in the figure below. (Figure is shown in the handout where x is catalyzing a reaction in which NADP+ becomes NADPH, in which glucose 6-phosphate becomes 6-phosphogluconate.) What would catalyze the reaction of Glucose 6-P to 6-phosphogluconate?

- (A) Glucokinase
- (B) Phosphoglucomutase
- (C) Glucose decarboxylase
- (D) Glucose dehydrogenase

*Bryan’s Insights:*

NADP is becoming NADPH. So the only atom being moved around here is hydrogen. So if literally all you saw was a hydrogen moving, then you’d pick (D). And this is the right answer.

Again, it’s really that easy. With enzymes, it can seem so complicated but the names tell us what they do. So glucose dehydrogenase pulls hydrogen off glucose and slaps it on NADP+ in this case.

For glucokinase, recognize the “kin” part of this where it moves a phosphate group around. Mutases change between isomers. Particularly, phosphoglucomutase is important in glycolysis, changing G1P to G6P. Decarboxylase does exactly what it sounds like where it takes off the carboxyl group to CO2 group. And of course, dehydrogenase pulls off hydrogen.

### [11:00] Question 29

Which of the following best describes the primary cellular energy source ATP?

- (A) ATP contains a pentose sugar and a purine base.
- (B) ATP contains a pentose sugar and a pyrimidine base.
- (C) ATP contains a hexose sugar and a purine base.
- (D) ATP contains a hexose sugar and a pyrimidine base.

*Bryan’s Insights:*

Walking into MCAT, this is something you need to know where you should be able to draw ATP. ATP is a nucleic acid derivative. So they’re all ribose sugar which are 5 carbon sugars.

The mnemonic you want to use here is Pure as Gold and Cut the Pie. Specifically, Pure (purine) As Gold (AG) and CUT the Pie (Pyrimidine). So A and G are purines and C, U, and T are pyrimidines. Hence, the right answer here is answer choice A.

### [12:40] Question 30

Many processes in living cells produce free radicals. All of these molecules can perform an antioxidant function *in vivo*, except:

- (A) Ubiquinone
- (B) Vitamin E
- (C) NADH
- (D) FAD

*Bryan’s Insights:*

NADH and FADH2 are classic cargo molecules where they can get oxidized to get produced. Ubiquinone is very important in the transport chain so you should know this. So it has to be C or D here.

NADH can lose its H or it could be FAD since it can pick up the H. In the whole Redox scheme where one gets oxidize and the other gets reduced, it’s got to be one of these two. In this case, the right answer is FAD because FAD gets reduced to FADH2. And somebody that gets reduced, oxidizes someone else. So FAD is an oxidant, not an antioxidant. So when you get reduced, you’re oxidizing somebody else. If you oxidize someone else, you’re an oxidant. And if you reduced someone else, you’re a reductant.

NADH reduces someone else, which would be the opposite of oxidizing someone else. So NADH could be called, theoretically, an antioxidant. Because an antioxidant is a molecule whose job is to *not *oxidize other people. But if it was just NAD, instead of NADH, then it would also be a reductant. So it’s really about having that H or not.